1. Technical Field
This disclosure relates generally to optimized photovoltaic systems. More specifically, the system disclosed herein relates to optimizing a short-string of photovoltaic panels to produce a consistent, reliable, and efficient source of power on residential rooftops.
2. Description of the Related Art
Photovoltaic systems convert solar energy into direct current electricity. A simple photovoltaic system may include a solar panel and a power inverter. As the solar panel in the simple photovoltaic system absorbs solar energy, the solar panel creates direct current electricity. The solar panel is typically connected to a power inverter to convert the direct current electricity into alternating current electricity to power electrical devices.
Conventional photovoltaic systems have several drawbacks. First, conventional photovoltaic systems require that each solar panel in a string of solar panels maintain the same azimuth. For example, in the United States, each solar panel in a string of solar panels should face south to absorb the maximum amount of solar energy available. However, on a residential rooftop, for example, space on south facing portions of a roof may be limited such that a homeowner cannot install a meaningful number of solar panels on a rooftop. For example, a rooftop on a residential home may be constructed with a hip roof (or a variant thereof), gables, or other vertical sides that prevent installation of one or more solar panels on a south facing portion of a rooftop. Homeowners with rooftops that limit solar panel installation on the desired azimuth are unable to efficiently take advantage of conventional photovoltaic technology.
In addition to maintaining a desired azimuth for solar panels in a photovoltaic system, the solar panels in such a system should further be installed at the same elevation angle relative to each other and the sun. Solar panels in photovoltaic systems generate direct current electricity in proportion to the amount of solar energy absorbed through the solar panels. In other words, a solar panel that is situated at an ideal angle to absorb solar energy will produce more direct current electricity than a solar panel that is situated at a non-ideal angle to absorb solar energy.
Such a configuration can be problematic in practice for two reasons. First, in many residential (and other) situations, it is difficult in practice to install every solar panel in a photovoltaic system at a consistent angle relative to every other solar panel and relative to the sun. Second, rooftop conditions on, for example, a residential rooftop may limit the available space for solar panels in a photovoltaic system to be installed at a consistent azimuth angle and elevation angle. For example, many residential roofs have a “split pitch” in which one section of roof has one pitch and another section of roof has another pitch. In other words, one section of roof on a residential house may be more or less steep than another section of roof on the residential house. In many cases, the varying steepness of a residential rooftop prevents multiple solar panels from being installed at a consistent elevation angle relative to the sun and at an angle that is consistent with other solar panels in the photovoltaic system. In some conventional technologies, power generation is limited by solar panels that cannot be placed at a consistent azimuth angle and elevation angle. In such technologies, each solar panel in a photovoltaic system generates power at the same level as the lowest level of power generated by a particular solar panel. In other words, if one solar panel generates less power because the solar panel is sub-optimally placed at an azimuth angle or an elevation angle that differs from other solar panels to which the sub-optimally placed solar panel is connected, every other solar panel, even if optimally situated, is limited in power production to the same level as the one sub-optimally situated solar panel. Accordingly, in some conventional technologies, a single sub-optimally situated solar panel can lower the power generation abilities of a photovoltaic system as a whole by reducing the power generation of every other solar panel in the photovoltaic system. Such a configuration limits the overall power that can be generated by the photovoltaic system and reduces the efficiency of the photovoltaic system.
Yet another drawback of conventional photovoltaic systems results from local shading on one or more solar panels in the photovoltaic system. For example, at least portions of residential rooftops may be shaded by vent stand pipes, satellite dishes, trees, chimneys, vent caps, television antennas, air conditioners, swamp coolers, other buildings, billboards, power poles, power lines, dirt, dust, and a number of other solar obstructions. In an exemplary photovoltaic system configuration, a large tree may fully or partially shade a portion of a roof throughout an afternoon. Because of the solar obstruction, a solar panel that is fully or partially shaded by the large tree would produce less direct current electricity than a solar panel that is not shaded. A photovoltaic system that has one or more solar panels in a string of solar panels that is fully or partially shaded drops the energy production level of the photovoltaic system to the level of power generation of the lowest performing solar panel. In other words, when one solar panel in a string of solar panels is shaded by a solar obstruction, power generation level for the entire string of solar panels is essentially the same as the power generation level of the single shaded solar panel.
Accordingly, because, for example, residential rooftops are relatively small, the available space for photovoltaic systems must be used efficiently to maximize power generation. Furthermore, for conventional photovoltaic systems, because every solar panel in the photovoltaic system should be positioned on a consistent azimuth, at a consistent elevation angle relative to each of the other solar panels in the system and the sun, and in an unshaded portion of a rooftop, conventional photovoltaic systems cannot efficiently take advantage of available space on a residential rooftop.
For example, FIG. 1 shows an implementation of photovoltaic system 100 that uses a traditional string topology. In this example, solar panels 105a, 105b, 105c to 105n (n solar panels) are connected to each other in a series electrical connection 110 and to string inverter 115. Because of the series electrical connection 110, if one of the solar panels is compromised by technical malfunction, change in azimuth, difference in elevation angle, or a solar obstruction, the entire string of solar panels 105a-105n is compromised. Similarly, a “solar panel mismatch” caused by manufacturing differences between solar panels, mixed types of solar panels, temperatures, and other issues can cause the string of solar panels in photovoltaic system 100 to operate at less than the desirable level. Thus, in addition to being prone to inefficient operation, traditional string topologies, such as photovoltaic system 100, are not ideal for residential use because residential rooftops, for example, have limited available space where a desirable and consistent azimuth angle and a consistent elevation angle can be obtained in an unshaded area of a residential rooftop.
FIG. 2 shows an implementation of photovoltaic system 200 using an optimized string topology. In this example, each of solar panels 205a, 205b, 205c to 205n (n solar panels) is directly connected to a respective one of maximum power point converters 220a, 220b, 220c to 220n (n maximum power point converters). Each of maximum power point converters 220a-220n is connected in a series electrical connection 210 to a string inverter 215. Essentially, conventional photovoltaic system 200 improves on conventional photovoltaic system 100 shown in FIG. 1 by providing maximum power point converters 220a-220n that monitor a maximum power point for their respective solar panel 205a-205n. In this way, maximum power point converters 220a-220n each independently adjust the current through a respective solar panel to maximize the power harvested from a respective solar panel, providing the available direct current electricity to string inverter 215, thus maximizing the electrical power available in photovoltaic system 200. For example, assume photovoltaic system 200 is installed on a residential rooftop. Solar panels 205a-205n absorb solar energy and produce direct current electricity which is conducted to their respective maximum power point converters 220a-220n. However, just after noon, a chimney begins to cast a shade on solar panel 205a. Maximum power point converter 220a, recognizes that the direct current electricity supplied by solar panel 205a is reduced because of the solar obstruction and independently adjusts its own operating characteristics to correspond with a shaded operating condition. Thus, unlike photovoltaic system 100, shown in FIG. 1, in which all solar panels' energy production would be compromised by the reduced output of solar panel 205a, photovoltaic system 200 operates more efficiently (than photovoltaic system 100 shown in FIG. 1). This is because the energy output of each of solar panels 205a-205n in photovoltaic system 200 is independently maximized on a per-panel basis by maximum power point converters 220a-220n. 
While photovoltaic system 200 is an improvement on photovoltaic system 100 shown in FIG. 1, photovoltaic system 200 introduces many more devices (maximum power point converters 220a-220n), each of which bring an additional capacity to fail and increase the financial cost of photovoltaic system 200 because for every new solar panel incorporated into a homeowner's system, the homeowner must also purchase an additional module to manage the new solar panel. Furthermore, even though photovoltaic system 200 includes maximum power point converters 220a-220n, photovoltaic system 200 still uses series electrical connection 210. Therefore, a failure of any of maximum power point converters 220a-220n, or any failure in electrical continuity along the string, would result in failure of that entire string to produce energy.
FIG. 3 shows an implementation of photovoltaic system 300 using conventional microinverter topology. In this example, each of solar panels 305a, 305b, 305c to 305n (n solar panels) is directly connected to a respective one of microinverters 315a, 315b, 315c to 315n (n microinverters). Each of microinverters 315a-315n in photovoltaic system 300 is connected in a parallel electrical connection as opposed to the series electrical connection described above with photovoltaic systems 100 and 200. Microinverters 315a-315n are configured to receive direct current electricity from solar panels 305a-305n and invert the direct current electricity to create alternating current electricity. Microinverters 315a-315n therefore output alternating current electricity into an alternating current bus (“AC bus”) 310. Microinverters 315a-315n may also monitor a maximum power point for a respective solar panel 305a-305n in order to maximize the electrical power available in photovoltaic system 300.
Photovoltaic system 300 enjoys at least one advantage over photovoltaic systems 100 and 200, described above with respect to FIGS. 1 and 2. Specifically, since photovoltaic system 300 is configured with each of solar panels 305a-305n and a respective each of microinverters 315a-315n in a parallel electrical connection, a failure in one of solar panels 305a-305n or microinverters 315a-315n will not compromise the electrical output of the entire system, unlike photovoltaic systems 100 and 200 described above with respect to FIGS. 1 and 2. At the same time, however, the use of microinverters 315a-315n requires that every solar panel in photovoltaic system 300 also directly connect to a microinverter. Thus, the financial cost of photovoltaic system 300 is much greater because a homeowner must buy a microinverter for each solar panel the homeowner intends to use. Furthermore, as in system 200, each additional microinverter that is included in photovoltaic system 300 provides an additional point of failure which is a particular concern given that microinverters 315a-315n contain electrical elements that are particularly susceptible to the extreme weather conditions that can be experienced on a rooftop. More specifically, because microinverters 315a-315n are used to invert direct current electricity to create alternating current electricity, they frequently contain electrolytic capacitors that can be sensitive to temperature, humidity, water, and other weather conditions that can be experienced on a rooftop. Microinverters 315a-315n are also complex, requiring more electrical components (than maximum power point converters 220a-220n of photovoltaic system 200, shown in FIG. 2) making microinverters 315a-315n more expensive in terms of equipment costs and less reliable, due to the large number of electrical components exposed to harsh conditions. Also, in attempting to prevent excessive reliability problems, these components must typically be specified as higher-grade versions able to withstand wider temperature extremes, which significantly increases costs. Thus, while the consequences of one of solar panels 305a-305n or corresponding microinverters 315a-315n failing are reduced by the parallel electrical connection of photovoltaic system 300, the need to supply a microinverter with every solar panel increases the number of devices in photovoltaic system 300 that can fail and affect system reliability, as compared with photovoltaic system 100. Photovoltaic system 300 is also more expensive, more complex, and more subject to reliability problems than photovoltaic system 200.
Microinverter technology in photovoltaic system 300 suffers from another drawback. As discussed above, each of microinverters 315a-315n invert direct current electricity supplied by solar panels 305a-305n into alternating current electricity. In the configuration described as photovoltaic system 300, this inversion of direct current electricity into alternating current electricity is inefficient for two reasons. First, because the direct current electricity output by solar panels 305a-305n is relatively low, each of microinverters 315a-315n must increase the voltage before the direct current electricity can be converted into alternating current electricity that is usable in an electrical device. However, because each voltage increase and waveform conversion is subject to inherent inefficiencies, microinverters 315a-315n introduce electrical losses into photovoltaic system 300. Second, the inherent inefficiencies of increasing voltage and converting direct current into alternating current are compounded in photovoltaic system 300 because each of microinverters 315a-315n is individually increasing voltage and converting the waveform independently of every other microinverter in photovoltaic system 300. Thus, less efficient inversions are performed by each of microinverters 315a-315n which are aggregated in the configuration of photovoltaic system 300. In other words, many small inefficient inversions performed by the microinverters 315a-315n sum to a total energy loss for photovoltaic system 300 that, on the whole, would be larger than the loss experienced by photovoltaic system 300 if a single inversion was to be performed on the aggregated current from the solar panels in a centralized inverter, such as in system 100.
FIG. 4 shows an implementation of photovoltaic system 400 using a conventional mini-inverter topology. In solar block 425a, each of solar panels 405a, 405b, 405c, and 405d is directly connected to boost converter 420a by DC electrical connections, as shown in FIG. 4. Similarly, solar panels 405e, 405f, 405g, and 405h are directly connected to boost converter 420b; solar panels 405i, 405j, 405k, and 405l are directly connected to boost converter 420c; and solar panels 405m, 405o, 405p, and 405q are directly connected to boost converter 420d. Each of boost converters 420a, 420b, 420c, and 420d is directly connected to a mini-inverter 415 via DC electrical connections 410. Photovoltaic system 400 may, to scale up the energy production of photovoltaic system 400, optionally add additional solar blocks such as solar block 425b, which are identical in topology and layout to solar block 425a, and connected in parallel to solar block 425a, as needed to suit a particular implementation.
Boost converters 420a-420d are similar to maximum power point converters 220a-220n shown in FIG. 2 and discussed above, except that each of boost converters 420a-420d have four maximum power point functions to monitor the maximum power independently per solar panel for four solar panels at the same time. This is an improvement over photovoltaic system 200 shown in FIG. 2, because photovoltaic system 400 uses fewer boost converters 420a-420d, and therefore has fewer points of failure in that portion of photovoltaic system 400 and has a lower overall complexity and potentially lower cost. However, photovoltaic system 400 is at a disadvantage when compared with photovoltaic system 200 in that photovoltaic system 400 increases the number of inverters needed, meaning the number of string inverters 215 in system 200, versus the number of mini-inverters 415 in system 400, for photovoltaic systems that include more than sixteen solar panels and, therefore, increases the overall system complexity and the number of failure points in that portion of photovoltaic system 400. Also, even though photovoltaic system 400 has fewer boost converters 420a-420d, photovoltaic system 400 still requires the same number of input connections from solar panels as in photovoltaic systems 200 and 300. Further, similar to system 300, the intended design of system 400 requires the mini-inverter to be located on the roof in that harsher environment, making it similarly susceptible to reliability issues.
It is therefore one object of this disclosure to provide a photovoltaic system that has the flexibility to minimize necessary electrical circuitry while, at the same time maximizing power generation in a specific area. It is a further object of this disclosure to take advantage of similar solar conditions in a particular area of a rooftop by co-locating a short-string of solar panels on a rooftop such that each solar panel in the short string shares a consistent azimuth angle and a consistent elevation angle.
It is a further object of this disclosure to provide a photovoltaic system that reduces system costs and has fewer points of failure than at least some other systems, is not incapacitated by a single point of failure, and that efficiently uses solar energy to create electrical power.